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Assembly process for out-of-plane mems and three-axis sensorsRelated Patent Categories: Semiconductor Device Manufacturing: Process, Packaging (e.g., With Mounting, Encapsulating, Etc.) Or Treatment Of Packaged SemiconductorAssembly process for out-of-plane mems and three-axis sensors description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070087474, Assembly process for out-of-plane mems and three-axis sensors. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001] The present application is related to U.S. Provisional Patent Application, Ser. No. 60/726,684, filed on Oct. 13, 2005, and Ser. No. 60/726,723, filed on Oct. 13, 2005, which are incorporated herein by reference and to which priority is claimed pursuant to 35 USC 119. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The invention relates to methods for the assembly of three dimensional microelectromechanical systems (MEMS). [0004] 2. Description of the Prior Art [0005] Microassembly techniques have been proposed for optical MEMS. In the publications listed below it is disclosed that a deposited layer of, for example, silicon carbide or silicon nitride serves as elastic flexures above a V-groove. Optical MEMS components can be inserted into the V-groove from the top and are held in place by the flexures. However, this method is only used for passive components. Further disclosure can be found in P. Boyle et.al., "Packaging solutions for MEMS/MOEMS using thin films as mechanical components", Proceedings of SPIE, vol. 4755, pp. 496-507, 2002, and R. Syms et.al., "Optical MEMS for telecoms", Materials Today, vol. 5, pp. 26-35, 2002 [0006] Three-axis sensors are often made by mounting three single-axis sensors perpendicularly on a machined cube-type structure. However, it is very difficult to perfectly align the three different sensors perpendicularly when using this technique. Also, all three sensors should ideally have the same center of gravity, which is very hard to obtain with macro-scale assembly. BRIEF SUMMARY OF THE INVENTION [0007] In the method of the illustrated embodiment of the invention, a sensor or accelerometer fabricated into an out-of-plane wafer or chip can be inserted into a cavity in a holder wafer, which in addition to holding the sensor in place also provides electrical conduction between the inserted sensor and the holder wafer or substrate. The use of SOI holder wafers allows for a more robust structure, since fingers for holding and contacting the out-of-plane wafer can be fabricated from the device layer in an SOI wafer, which fingers are much thicker than the very thin layer of silicon nitride or silicon carbide in the prior art assemblies. [0008] The illustrated embodiment of the invention is thus a method for assembly of three-dimensional micromachined devices. By etching a cavity in silicon-on-insulator (SOI) wafer and placing a second chip or wafer vertically in the cavity, a three-dimensional structure is obtained. In addition to allowing for mounting of a device perpendicular to a surface of the SOI wafer, this process also enables post-fabrication assembly of three-axis sensors. [0009] When compared to macro-scale mounting of a sensor perpendicular to a surface (or relative to another sensor), the micro-scale assembly technique of the illustrated embodiment is easier to control. Since the angles and cavities are defined with photolithography, any deviation from perpendicularity is determined mainly by fabrication imperfections. In contrast, the precision of prior art macro-scale assembly relies on machining of the mounting block and the skill of the assembler. [0010] The micro-scale assembly process also allows for mounting of three sensors very close together, which means that the center of gravity of each sensor is very close to the center of gravity of the other sensors, which is a desirable property in three-axis sensors. [0011] Thus, it can be understood that the illustrated embodiment of the invention can be used to mount in-plane sensors, or passive components, perpendicular to a surface. It also enables the assembly of three-axis sensors. For example, three-axis accelerometers can be manufactured with this process, which can be used for acceleration measurements, including, but not limited to, the following applications: inertial navigation systems, impact testing, and structural monitoring. [0012] More particularly, the illustrated embodiment of the invention is characterized as a method of assembling a three dimensional micromachined structure comprising the steps of: defining a cavity in a holder wafer having a thick upper layer; providing a plurality of fingers in the thick upper layer extending from the holder wafer into the cavity; and disposing an out-of-plane wafer into the cavity in the holder wafer in engagement with the fingers to hold the out-of-plane wafer in place in an out-of-plane position with respect to the holder wafer. [0013] In one embodiment the method further comprises the step of providing an electrical connection through the fingers between the out-of-plane wafer and the holder wafer. [0014] In another embodiment the step of disposing the out-of-plane wafer into the cavity in the holder wafer further comprises affixing the out-of-plane wafer in the cavity of the holder wafer to provide a robust structure. [0015] In still another embodiment the method further comprises the step of defining at least one groove in the out-of-plane wafer for engagement with the fingers and where disposing the out-of-plane wafer into the cavity in the holder wafer comprises sliding the fingers into the groove. [0016] The step of defining at least one groove in the out-of-plane wafer for engagement with the fingers comprises defining two grooves in the out-of-plane wafer, and where disposing the out-of-plane wafer into the cavity in the holder wafer comprises sliding the fingers into the two grooves. Preferably, the step of defining two grooves in the out-of-plane wafer comprises defining two opposing grooves in the out-of-plane wafer. [0017] The step of providing the plurality of fingers in the thick upper layer extending from the holder wafer into the cavity comprises providing fingers in the thick upper layer extending from the holder wafer into the cavity. The fingers may be provided on a single side or two opposing sides of the cavity in the thick upper layer. [0018] The illustrated embodiment of the invention is still further characterized as a method of assembling a three dimensional microelectromachined structure (MEMS) including active MEMS components in a three dimensional assembly comprising: etching a cavity in silicon-on-insulator (SOI) holder wafer having a thick device layer; engaging an out-of-plane wafer including active MEMS devices with the holder wafer to vertically extend the out-of-plane wafer from the holder wafer, engagement of the out-of-plane wafer and holder wafer causing the out-of-plane wafer and holder wafer to be orthogonally oriented with respect to each other; and electrically coupling the out-of-plane wafer and holder wafer together to obtain a three-dimensional active MEMS assembly. [0019] The step of electrically coupling the out-of-plane wafer and holder wafer together comprises providing a plurality of fingers extending from the holder wafer onto conductors on the out-of-plane wafer, and engaging the out-of-plane wafer with the holder wafer with the fingers to hold the out-of-plane wafer in place in an out-of-plane position with respect to the holder wafer and to electrically couple conductors on the out-of-plane wafer with conductors on the holder wafer. [0020] The step of engaging the out-of-plane wafer into the cavity to extend vertically from the cavity comprises positioning three accelerometers close together with a negligible distance between the center of gravity of each of the three accelerometers. [0021] The invention also includes within its scope and embodiments an apparatus made according to any combination of the above method steps and/or the structure of the apparatus which is fabricated from any combination of those method steps. 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